Metal nanoparticles exhibit a large potential for the development of innovative and cost-effective sensing devices. They fulfill key requirements for biosensors such as the potential for miniaturization as well as for high parallelization, and they are compatible with the molecular world for the required biofunctionalization approaches. Their optical properties based on the localized surface plasmon resonance (LSPR) are well adjustable from the UV- to the infrared spectral range using chemical synthesis. Due to the strong influence of the surrounding dielectrics on the resonant properties these particles offer a high potential for sensing of minimal changes in the surrounding media. Additionally, plasmon nanoparticles can induce a local field-enhancement and so a signal amplification such as for fluorescence or Raman-spectroscopy. In general, plasmon nanoparticles are well suited as label or as transducer for different optical detection techniques. We will give an overview about recent developments in this field, and will present different sensing strategies at single particle or ensemble level and based on planar or fiber-based systems aiming for ultrasensitive point-of care applications in bioanalytics.